Lanthanide-Boosted Singlet Oxygen from Diverse Photosensitizers along with Potent Photocatalytic Oxidation

Singlet oxygen (1O2) plays a central role in photochemical and photobiological research. Although many photosensitizers for efficient 1O2 generation were reported, further improving its yield and oxidation power is still highly desirable. Instead of developing new ones, current photosensitizers might be boosted by mediators to facilitate energy transfer. Taking advantage of the long triplet state lifetime of lanthanide ions (Ln3+), we herein demonstrate their roles as potent oxidation mediators. Using oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) at neutral pH as a difficult model reaction, based on the fluorescence, phosphorescence, and metal-binding properties, various dyes and nanomaterials were classified into four types. The 1O2 emission of carbon dots and rose bengal was enhanced 4 times in the presence of Ce3+. Some nonphosphorescent, but strongly fluorescent dyes that are not known as photosensitizers can still be mediated by Ln3+ to produce 1O2, but metal-chelating calcein was not enhanced. Finally, nonemissive dyes failed to show activity. As mediators, the excited Ln3+ can migrate a long distance and transfer energy to O2, resulting in high 1O2 yield. Since redox-active Ce3+ and Eu3+ had the highest activity, participation of oxidation involving excited lanthanides might be possible too. In addition, Ln3+ also enhanced the activity of graphene quantum dots, graphene oxide, and g-C3N4. Rapid degradation of organic dyes was demonstrated, further supporting a high photocatalytic activity of the Ln3+-mediated system.